In recent years, particulates, contained in exhaust gases that are discharged from inner combustion engines of vehicles, such as buses and trucks, and construction machines, have raised serious problems since those particulates are harmful to the environment and the human body. For this reason, various ceramic filters, which allow exhaust gases to pass through porous ceramics and collect particulates in the exhaust gases to purify the exhaust gases, have been proposed.
Normally, the ceramic filter of this type has a structure in that a number of through holes are arranged side by side in one direction and partition wall that separate the through holes from each other are allowed to function as filters.
In other words, each of the through holes formed in the ceramic filter is sealed with a filler at either of ends of its exhaust gas inlet side or outlet side so that exhaust gases that have entered one through hole are discharged from another through hole after having always passed through the partition wall that separates the through holes; thus, when exhaust gases are made to pass through the partition wall, particulates are captured by the partition wall so that the exhaust gases are purified.
As such a purifying process for exhaust gases progresses, particulates are gradually accumulated on the partition wall that separates the through holes of the ceramic filter to cause clogging and the subsequent hindrance in gas permeability. For this reason, the above-mentioned ceramic filter needs to be subjected to a regenerating process regularly by burning and removing the particulates that cause the clogging by the use of a heating means such as a heater.
In such a regenerating process, however, it is difficult to evenly heat the ceramic filter, with the result that heat is locally generated due to the burning of the particulates to cause a large thermal stress. Moreover, even during normal operations, an uneven temperature distribution occurs inside the ceramic filter due to a thermal impact or the like derived from an abrupt temperature change in the exhaust gases, resulting in a thermal stress.
Consequently, in the case where the ceramic filter is constituted by a single ceramic member, cracks tend to occur to cause a serious problem in collecting the particulates.
Moreover, in an attempt to produce a large-size ceramic filter, since sintering shrinkage becomes greater upon sintering, it becomes difficult to control the shape.
For this reason, a honeycomb filter having the following structure has been proposed: a ceramic filter is divided into a plurality of porous ceramic members, with a number of through holes formed therein, and the porous ceramic members are combined with one another through adhesive layers (for example, see JP Kokai Hei 8-28246 and JP Kokai 2001-190916).
With the honeycomb filter of this type, it becomes possible to reduce a thermal stress that is exerted on the honeycomb filter during regenerating processes and operations, and also to freely adjust the size thereof by increasing or reducing the number of the porous ceramic members.
However, in the conventional honeycomb filter having this structure, it has been considered that it is desirable to set the thermal expansion coefficient of the porous ceramic member and the thermal expansion coefficient of the adhesive layer in the same level.
The reason for this is explained as follows. Actually, the above-mentioned honeycomb filter is used in a wide temperature range, for example, from 10 to 800° C., and when the thermal expansion coefficient of the porous ceramics member is different from the thermal expansion coefficient of the adhesive layer, cracks tend to occur in the porous ceramic member and the adhesive layer due to the difference in the coefficients of these members.
However, in the case where the thermal expansion coefficient of the porous ceramic member and the thermal expansion coefficient of the adhesive layer are made completely identical to each other, this case is the same as the case using a single ceramic member; therefore, when particulates are burned locally in the honeycomb filter, that is, when a local temperature change occurs therein, due to: uneven amounts of accumulated particulates; uneven amounts of catalyst upon allowing the honeycomb filter to bear the catalyst and unevenness of heat applied by a heater; exhaust gases and the like, a great thermal stress is generated between the portion having this local temperature change and the other portions, with the result that cracks tend to occur in the porous ceramic member and the adhesive layer.
Moreover, in recent years, in order to quickly raise the temperature inside the honeycomb filter by the heat of exhaust gases, methods for reducing the thermal capacity of the honeycomb filter has been examined. In the case where a catalyst that is capable of purifying CO, HC, NOx and the like in exhaust gases is supported on the honeycomb filter having such a low thermal capacity, since the temperature of the honeycomb filter is easily raised to a catalyst-active temperature by using high-temperature exhaust gases and the like, the above-mentioned honeycomb filter can also be used desirably as a catalyst supporting body.
Furthermore, in an attempt to prevent a pressure (back pressure) imposed at the portion from the engine to the honeycomb filter from becoming too high even when the engine is driven with a large amount of catalyst supported on the honeycomb filter, techniques for increasing the porosity of the honeycomb filter have been considered.
In the case of such honeycomb filters having a low thermal capacity and a high porosity; however, since the density is low and the strength is poor, the resulting problem is that the honeycomb filter tends to be damaged in the manufacturing processes to cause low yield. Moreover, even in the case of those filters obtained without damages in the manufacturing processes, when a local temperature change occurs therein, due to uneven amounts of accumulated particulates, uneven amounts of catalyst upon allowing the honeycomb filter to bear the catalyst and unevenness of heat applied by a heater, exhaust gases and the like, a great thermal stress is exerted between the portion having this local temperature change and the other portions, with the result that cracks tend to occur in the porous ceramic member and the adhesive layer.
Moreover, in an attempt to prevent exhaust gases from leaking from the peripheral portion when the honeycomb filter is placed in an exhaust gas passage of an inner combustion engine, upon manufacturing a honeycomb filter, a surface treatment is normally carried out in which, a coating material layer is formed on the circumferential portion thereof by using a coating material to fill the through holes exposed by the cutting processes or the like, after the shape of the circumference of the honeycomb filter has been formed into a cylinder shape or the like through cutting processes or the like.
Conventionally, with respect to the coating material to be used in the surface treatment, JP Kokai 2000-102709 has disclosed a material that contains at least inorganic fibers, an inorganic binder, an organic binder and inorganic particles.
Here, in order to manufacture a honeycomb filter having a low thermal capacity and a high porosity, there have been demands for a coating material layer that has a low thermal capacity and a superior heat-insulating property, and is capable of alleviating a thermal stress that has been generated, to reinforce the honeycomb filter; however, conventional coating materials have been failing to form a coating material layer that can sufficiently satisfy these characteristics.
Moreover, it has been found that, with respect to honeycomb filters having various external shapes in the cross-section, that is, not only a rectangular shape but also a round shape and an elliptical shape, the formation of the coating material layer after shape-machining process such as cutting causes considerable degradation in the outside dimensional precision.